Long-term holding of available strains of inbred mice and rats to define and characterize the incidence of spontaneous neoplasms and its relationship to the host's natural type C virus expression and natural immune defenses. Determine the natural incidence of type C RNA virus expression in neoplastic and normal tissues as determined by serological detection of antigens and in vivo and in vitro isolation of infectious virus from tissues of rodents with and without spontaneous neoplasia, and characterization of isolates by in vivo and in vitro assays for biological activity and properties. Production of vaccines used for immunotherapy and immunoprevention of spontaneously occurring tumors. Vaccines to include viral, cellular, and subcellular preparation. Monitor and define the quality of the vaccine preparations. Define optimal conditions for production, handling, and storage of vaccines and define optimal route, dose, and schedule of immunization.